CR 20:9-17 (2002) - doi:10.3354/cr020009

The North Atlantic Oscillation influence on Europe: climate impacts and associated physical mechanisms

Ricardo M. Trigo1,2,*, Timothy J. Osborn1, João M. Corte-Real3

1Climatic Research Unit, University of East Anglia, Norwich NR4 7TJ, United Kingdom 2Department of Physics, and 3Institute for Applied Science and Technology (ICAT), Faculty of Sciences, University of Lisbon, 1749-016 Lisbon, Portugal

ABSTRACT: A multivariable analysis of the influence of the North Atlantic Oscillation (NAO) on the climate of the North Atlantic and European sectors is presented using the 40 yr (1958-1997) consistent data set from NCEP. Using high and low NAO index
composites, anomaly fields of climate variables are then interpreted based on physical mechanisms associated with the anomalous mean flow (characterised by the surface wind field) and the anomalous eddy activity (characterised by the surface vorticity and
the 500 hPa storm track fields). It is shown that NAO-related temperature patterns are mainly controlled by the advection of heat by the anomalous mean flow. However, large asymmetries between minimum and maximum temperatures, and more significantly,
between positive and negative phases of NAO imply the importance of a different mechanism, namely, the modulation of short wave and long wave radiation by cloud cover variations associated with the NAO. Furthermore, NAO influence on 2 different
precipitation-related variables‹precipitation rate and precipitable water‹displays different patterns. Precipitable water is shown to be strongly related to the corresponding anomaly fields of temperature while precipitation rate appears to be controlled
by the surface vorticity field and associated strength of the tropospheric synoptic activity. Erratum